This invention relates to an image pickup device using a solid-state imaging element, such as a CCD, that converts the picked-up optical image into an electric signal and an exposure control method in the image pickup device, and more particularly to an image pickup device capable of alleviating smears and an exposure control method in the image pickup device.
In recent years, electronic image pickup devices for converting the picked-up optical image into an electric signal have been widely used. Of such electronic image pickup devices, digital cameras for picking up still pictures are beginning to have the performance comparable to silver-bromide analog cameras. In this specification, unless otherwise specified, explanation will be given on the assumption that an electronic image pickup device is an electronic still camera with a digital circuit system, or a digital camera. The techniques of the present invention can be applied to analog electronic image pickup devices, or analog electronic still cameras. In the explanation, however, a case where the present invention is applied to a digital camera will be explained, taking the digital camera as a typical electronic image pickup device.
The still picture camera has been required to have a wide variety of functions. Of the functions, exposure control in photography has been especially given importance. The still picture camera has realized the various exposure functions achieved by the silver-bromide camera and further accomplished functions the silver-bromide camera cannot realize.
One of the functions the silver-bromide camera cannot realize is an electronic shutter realized by controlling the accumulation of charges in a CCD image pickup element. The electronic shutter can realize a high-speed shutter function the mechanical shutter of an ordinary silver-bromide camera cannot realize. To make effective use of the function, a progressive scanning (sequential scanning) CCD image pickup element has been incorporated into a digital camera. On the other hand, there are various disadvantages stemming from the image pickup element, including the factor degrading the picture quality. To prevent the disadvantages from becoming tangible, various improvements have been made in the digital camera for practical use.
A typical one of the disadvantages is what is called a smear phenomenon that occurs when intense light rays strike the CCD image pickup element. Although the smear phenomenon is a phenomenon including not only smears but also deterioration of picture quality caused by smears and blooming, the phenomenon is called just a smear in this specification according to usage among those skilled in the art. Specifically, the phenomenon takes place when charges not trapped in the original charge accumulation region climb over the potential barrier and leak into the vertical transfer channel and some diffraction components or multiple reflection components of the incident rays leak into the vertical transfer channel under the shade film. When ordinary steady light rays cause a smear phenomenon, the phenomenon lasts throughout a vertical transfer period. Thus, in the case of light rays considered to cause the phenomenon, for example a spotlight, vertical stripes extending upward and downward appear on the picked-up image, which impairs the picture quality seriously. Several methods of alleviating the smear phenomenon have been proposed, including a method of correcting the image signal and a method of using an optical shutter. The method of correcting the image signal, however, gives no basic solution to the problem of the smear phenomenon, because it corrects the image signal after having obtained the electronic image signal.
In contrast, one known method of using an optical shutter employs a driving method as shown in FIGS. 1A to 1D. FIGS. 1A to 1D show an example of timing charts for driving a CCD image pickup element with a sequential scanning overflow drain structure: FIG. 1A shows the opening and closing of a mechanical shutter; FIG. 1B shows a transfer gate pulse for transferring a charge signal from a charge accumulation region to a vertical transfer channel, or a TG (transfer gate) signal; FIG. 1C shows a high-voltage pulse applied to the substrate to force the charges in the charge accumulation region to be discharged into the semiconductor substrate (=vertical overflow drain VOFD), or a VSUB signal; and FIG. 1D shows a driving voltage signal for driving the vertical transfer channel (vertical CCD, also referred to as the VCCD), or a VCCD signal.
With the method shown in FIGS. 1A to 1D, in response to an image pickup instruction (not shown), the semiconductor substrate starts to be driven by the VSUB signal and the vertical transfer channel (VCCD) starts to be driven at high speed by the VCCD signal. Because the VSUB signal is used to discharge the charges from the accumulation region, only the final output pulse related to the exposure timing is effective. To discharge the charges sufficiently and stabilize the potential in the element, H-rate driving is effected so as to output a short-time pulse with a specific width in each horizontal blanking period. In FIG. 1C, time t4 when the final VSUB pulse represented by a bold line was outputted corresponds to the exposure start time.
On the vertical transfer channel, continuous high-speed driving is done to discharge the unnecessary charges in the transfer channel, staring at a suitable time before the exposure start. Differently from the normal driving mode in which the individual pixel signals are read separately (in this mode, a vertical driving pulse corresponding to one unit=one horizontal pixel in each horizontal blanking interval), the vertical transfer channel is driven continuously at a speed several times or tens of times as fast as that of the normal driving. Consequently, the unnecessary charges, including smears, occurring as a result of the light rays continuing to strike the image pickup surface are discharged at high speed. The high-speed driving is continued until immediately before time t5, which is the timing of a subsequent exposure end. If a driving multiple (defined as the reciprocal ratio of the time required to transfer one screen) in high-speed driving with respect to normal driving is X, the high-speed driving period requires a minimum of 1/X of one frame period.
When a TG pulse is outputted at time t5, the accumulated optical charges are transferred to the shaded vertical transfer channel. At time t5, the exposure is completed. Then, the mechanical shutter is closed immediately to prevent smears. Since a delay time dt of about several hundreds of microseconds to several milliseconds is required for the mechanical shutter to close, the reading of signals in normal driving by the VCCD signal is not started in the period dt. At the time when the mechanical shutter is closed, actually at time t6 including a margin taking into account variations in the operation of the mechanical shutter, the reading of image signals in normal driving is started. Then, the mechanical shutter is kept closed until the read period for one frame in which the transfer of at least one frame of image signals is completed has elapsed.
As described above, when the signal is read, no smear takes place because no light ray strikes the image pickup element. In addition, since the charge accumulation time is completely controlled electrically, the high-speed shutter almost impossible to realize mechanically can be realized without being affected by variations in the mechanical shutter. An example of such techniques has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-191170.
The aforementioned conventional exposure control has the following problem: a smear caused by the light rays projected immediately before at least the exposure period Ts ends, or immediately before a TG pulse is outputted, cannot be discharged completely even by high-speed discharging. When it is assumed that the vertical transfer channel is driven at high speed X times as fast as that of normal driving as described above, a time of 1 Tfr/X is required to transfer a screen of charges, where Tfr is one frame period. As a result, smears occurred at one end of the screen (or smears occurred in the center of the screen in the half of one frame period) immediately before a TG pulse is generated cannot be discharged completely and remain in the transfer channel, with the result that they become tangible as vertically striped white smears.
This problem becomes more significant as much more pixels are used in the image pickup element. The reason is that use of an enormous number of pixels tends to make the transfer time (frame rate) of the transfer channel longer, which lengthens the required discharging time 1 Tfr/X in high-speed transfer accordingly.